Transcarboxylase from propionic acid bacteria and pyruvate carboxylase from animal tissues and from microorganisms is the subject of these studies. These are biotin enzymes, the first catalyzes the transfer of a carboxyl from methylmalonyl CoA via the biotinyl group of the enzyme to pyruvate yielding oxalacetate and propionyl CoA and the second, the fixation of CO2 on the biotinyl group of the enzyme and then transfer to pyruvate to yield oxalacetate. Transcarboxylase consists of three different types of subunits. The keto acid sites are on peripheral subunits, the CoA ester sites are on a central subunit and the carboxyl group is transferrred between these sites via the biotinyl subunit. The amino acid sequence of the carboxyl carrier protein has been determined and the portion identified which is involved in binding together of the peripheral and central subunits as well as the portion involved in placing the biotinyl group near the substrate sites on the two subunits. Pyruvate carboxylase of animal tissue, in contrast to transcarboxylase, is made up of four identical large peptide chains, each containing a biotin and the substrate sites. In contrast, pyruvate carboxylase of Pseudomonas citronellolis and Azotobacter vinelandii contain a biotinyl polypeptide and another polypeptide. Thus the functional unit in these biotinyl enzymes contains two parts in contrast to transcarboxylase with three parts and the animal enzyme with only one part. Cross-linking with dimethyl suberimidate indicates the P. citronellolis enzyme may have a a2 b2 structure. Studies of the activation of the animal pyruvate carboxylase by acetyl CoA have been continued. It is now possible to prepare pyruvate carboxylase with no deacylating activity, this greatly facilities these studies. BIBLIOGRAPHIC REFERENCES: Utter, M. F., Barden, R. E. and Taylor, B. L., Adv. in Enzymology 42, 1-72 (1975) "Pyruvate Carboxylase: An Evaluation of the Relationships Between Structure and between Structure and Catalytic Activity." Fung. C. H., Zander, G. and Utter, M. F., Fed. Proc. Absts. (1975 p. 589 Abst. #5098.